1. Field of the Invention
The present invention relates to a rack guide of synthetic resin which is housed in a steering gear box of a rack and pinion type steering device of a vehicle for transforming rotations of a steering shaft around its axis into right and left movements of a tie rod.
2. Description of the Prior Art
A conventional rack and pinion type steering device is disclosed in Japanese Utility Model Publication SHO 57-17196. In this publication, as shown in FIGS. 7 and 8, a pinion 102 connected to a steering shaft side member and a rack bar 104 connected to a tie rod side member have teeth which are mesh-engaged with each other, whereby rotations of the steering shaft are converted to right and left movements of a tie rod. The rack bar 104 moves reciprocally inside a housing 106 and is slidably supported by a rack guide 108 which is positioned opposite to the pinion 102. The rack guide 108 is slidably inserted within the housing 106 and is pushed toward the pinion 102 by a spring 112 which is interposed between the rack guide 108 and a plug 110.
Functions of the rack guide 108 are to push the rack bar 104 against the pinion 102 with such a suitable pre-load as to insure the mesh-engagement between the pinion 102 and the rack bar 104 and to suppress rolling of the rack bar 104, which may otherwise occur due to the manufacturing tolerances of the meshing teeth, during the right and left movements of the rack bar 104.
Conventionally, a sintered rack guide has been mainly used. However, with the sintered rack guide, there has been a problem of noises being generated due to a jittering of the rack guide, such jittering arising due to the clearance between the sintered rack guide and the housing.
Japanese Patent Publication SHO 59-216764 discloses a noise suppressing structure. As shown in FIGS. 9 and 10, the rack guide 114 of SHO 59-216764 incorporates in a wall of the rack guide a pair of spaces 116 and 118 extending in a circumferential direction whereby the wall of the rack guide 114 is divided into an inner cylinder 124 and an outer cylinder 126 which are connected to each other at two positions 120 and 122 in the circumferential direction of the rack guide. In the rack guide 114, the outer cylinder 126 continuously extends over the entire circumference of the rack guide. The outer cylinder 126 is manufactured in a shape of ellipse and is inserted into the housing 128 so that a clearance between the rack guide and the housing is made zero.
Another noise suppressing structure is disclosed in the previously-mentioned Japanese Utility Model Publication SHO 57-17196. Referring again to FIGS. 7 and 8, the rack guide 108 of SHO 57-17196 includes a single cylindrical wall 130 in which a slit 132 extending in an axial direction is formed. Owing to the slit 132, the rack bar supporting surface 134 of the rack guide 108 can be spread to decrease a clearance between the housing 106 and the rack guide, thereby suppressing the generation of noises.
However, in the above-mentioned noise suppressing structures, there still remain the following problems.
Though the rack guide 114 of SHO 59-216764 has excellent noise suppression effects, the rack guide 114 has the defect that the interference between the rack guide 114 and the housing 128 increases in proportion to an increase in the ambient temperature due to the thermal expansion differences between the two so that, in a worst case, the rack guide 114 locks with the housing 128. In such a locking state, a steering load of a driver will be increased when the locking has occurred during a time when the rack guide is strongly pushed against the rack bar and, conversely, noises will be caused when the locking has occurred during a time when the rack guide is away from the rack bar.
Recently, the space in the engine room of a vehicle has been designed small for the purpose of making it compact. Therefore, the rack guide is often exposed to high ambient temperatures and the above-mentioned locking raises a severe problem.
In the rack guide 108 of SHO 57-17196, since the rack bar supporting surface 134 itself is spread, axial movement of the rack guide 108 due to manufacturing tolerances of the teeth of the rack and the pinion can not be absorbed, although a radial clearance between the rack guide 108 and the housing 106 becomes zero. As a result, smooth rotation of the pinion 102 and smooth rotation of the steering wheel can not be obtained.